Aircraft navigation systems often include receivers for beacon signals which provide directional information in flight. For example, nearly every civilian aircraft has at least one receiver for a common type of navigation beacon, called a VHF Omni Range (VOR) beacon, which transmits signals that provide information about the aircraft's bearing with respect to the transmitting beacon. Flight paths between two points are plotted in light of VOR beacon locations. The overall flight path from destination to arrival may comprise several flight-path segments between waypoints, and it may pass through the effective range of several navigation beacons.
There are a number of methods used to plot flight paths between selected points. In one such method, the aircraft simply flys from one VOR beacon toward the next beacon generally along its overall flight path. This kind of navigation produces a zigzagging flight path between beginning and end points, which wastes both time and fuel. Crowding problems may occur around major airports if a large number of aircraft are flying toward the same beacon. Also, small airports typically are located away from the VOR beacons, making navigation to them difficult.
Area navigation is a navigational system that permits a pilot to fly directly between any two arbitrary waypoints, thus freeing him from the requirement of flying along courses between VOR beacons and avoiding the problems discussed above. To use area navigation, one must know the aircraft's present flight position with respect to the desired waypoint. If the bearings and ranges between the aircraft and a nearby VOR, and between the VOR and waypoint, are known, the course and range to the waypoint then can be determined using simple navigational triangles (the law of sines).
Generally the bearing and range between a VOR and a preselected waypoint may be determined from navigational maps. The VOR equipment on board can be used to estimate the bearing from the plane to the nearby VOR.
Determining the range from the plane to the VOR is more difficult. Larger aircraft have relatively expensive distance measuring equipment (DME) to determine the distance between the aircraft and a given VOR. DME sends a signal to a DME receiver at the VOR beacon, which sends a return signal to the aircraft. The aircraft DME measures the echo time, from which the aircraft's distance to the beacon may be derived. Once the aircraft-to-beacon bearing and range are known, the plane's course and range to the desired waypoint may be derived.
DME is relatively expensive, and is out of the reach of many small aircraft users. Also, it is an "active" system, requiring action on the part of equipment at the beacon. Therefore, if a large number of aircraft are using DME, they have to wait for service at crowded beacons such as those located near large airports.
Another method of calculating the course and range to a waypoint is to track the aircraft's bearing with respect to two VOR beacons and find the point where the two lines intersect. This method requires two VOR receivers and extensive calculations; and is of little value if the aircraft is not continually within range of two VOR beacons. (A beacon's range is about 50 nautical miles for aircraft at 2500 feet.)
Yet another method is to compare the aircraft speed to the rate of change of the aircraft's bearing with respect to a VOR beacon off of the aircraft wingtip. This again requires complex calculations, making recovery difficult once lost.
Finally, there is apparatus (such as the DCE 400 marketed by Collins) which requires navigation directly toward a first VOR beacon and calculates the range to that beacon using a second VOR signal beacon.